It is an object of the present invention to provide an optical amplifying unit to be used for optical telecommunications. The invention also relates to an optical transmission system, more particularly a wavelength division multiplexing (WDM) optical transmission system, which uses the above-mentioned optical amplifying unit. The optical amplifying unit of the invention is also adapted to be used in analog CATV systems.
In WDM optical transmission systems, transmission signals including several optical channels are sent over a same line, that can include one or more optical amplifiers, by means of wavelength division multiplexing. The transmitted channels may be either digital or analog and are distinguishable because each of them is associated with a specific wavelength.
Present-day long-distance high-capacity optical transmission systems use optical fiber amplifiers that, differently from previously used electronic regenerators, do not need DE/EO conversion. An optical fiber amplifier includes an optical fiber of preset length, having the core doped with one or more rare earths so as to amplify optical signals by stimulated emission when excited by pump radiation. This pump radiation, when injected into the active fiber, excites the ions of the rare earth element, leading to gain in the core for an information bearing signal propagating along the fiber.
Rare earth elements used for doping typically include Erbium (Er), Neodymium (Nd), Ytterbium (Yb), Samarium (Sm), Thulium (Tm) and Praseodymium (Pr). The particular rare earth element or elements used is determined in accordance with the wavelength of the input signal light and the wavelength of the pump light. For example, Er ions would be used for input signal light having a wavelength of 1.55 xcexcm and for pump power having a wavelength of 1.48 xcexcm or 0.98 xcexcm; co-doping with Er and Yb ions, further, allows different and broader pump wavelength bands to be used.
Optical fibers doped with erbium (Er) have been developed for use as both optical amplifiers and lasers. These devices are of considerable importance since their operating wavelength coincides with the third window for optical fiber communications, around 1550 nm. EP patent application Ser. No. 98110594.3 in the name of the Applicant proposes a thirty-two channels WDM optical transmission have optical characteristics that allow, in addition to the required wavelength selectivity, a predetermined spatial distribution of the light. If a micro-optic coupler is used, a focusing lens system able to provide the considered spatial distribution of the light is very difficult to implement. Therefore, the use of a double-cladding active fiber involves difficulties in achieving a high coupling efficiency between the pump source and the active fiber. Moreover, the considered micro-optic coupler has a relatively high insertion loss, greater than 1 dB at 1550 nm. system that uses erbium-doped fiber amplifiers (EDFAs) in the wavelength bands 1529-1535 nm and 1541-1561 nm.
Several methods have been proposed to improve the system performances, for example in terms of amplification gain and amplification bandwidth.
One technique for improving the system performances consists in co-doping an erbium-doped amplification fiber with ytterbium (Yb). Co-doping an active fiber with erbium and ytterbium not only broadens the pump absorption band from 800 nm to 1100 nm, offering greater flexibility in selection of the pump wavelength, but also greatly increases the ground state absorption rate due to the higher absorption cross section and dopant solubility of ytterbium. The ytterbium ions absorb much of the pump light and the subsequent cross relaxation between adjacent ions of erbium and ytterbium allows the absorbed energy to be transferred to the erbium system. As described in Grubb et al., xe2x80x9c+24.6 dBm output power Er/Yb co-doped optical amplifier pumped by diode-pumped Nd:YLF laserxe2x80x9d, Electronics Letters, 1992, 28, (13) pp. 1275-1276, and in Maker, Ferguson, xe2x80x9c1.56 xcexcm Yb-sensitized Er fibre laser pumped by diode-pumped Nd:YAG and Nd:YLF lasersxe2x80x9d, Electronics Letters, 1988, 24, (18), pp. 1160-1161, the co-doping technique may be applied to efficiently excite fiber amplifiers and lasers through direct pumping in the long wavelength tail of ytterbium absorption spectrum. This pumping is preferably performed by means of diode-pumped solid state lasers, for example 1047 nm Nd:YLF lasers or 1064 nm Nd:YAG lasers.
Using an erbium and ytterbium co-doped amplification fiber to amplify communication signals is further described in EP 0 803 944 A2 and in U.S. Pat. No. 5,225,925. EP 0 803 944 A2 refers to a multistage Er-doped fiber amplifier (EDFA) operating in the wavelength band 1544-1562 nm and comprising a first stage that includes Er and Al and a second stage that includes Er and a further rare earth element, for example Yb. Such multistage EDFA can have advantageous characteristics in the cited wavelength band over the all-erbium amplification systems, e.g. a relatively wide flat gain region, and relatively high output power, without significant degradation of the noise figure. However, the Applicant noted that the amplifier proposed in EP 0 803 944 A2 offers no advantages in terms of number of transmitted channels, the amplification bandwidth being still limited to the relatively narrow (and largely exploited) 1544-1562 nm band. Furthermore, the Er/Yb second stage is pumped by means of a diode-pumped Nd-doped fiber laser emitting at 1064 nm. This pump source, largely used for the excitation of mono-modal amplification fiber, is relatively expensive and bulky.
U.S. Pat. No. 5,225,925 relates to an optical fiber for amplifying or sourcing a light signal in a single transverse mode. The fiber comprises a host glass doped with erbium (Er) and a sensitizer such as ytterbium (Yb) or iron (Fe). Preferably the host glass is a doped silica glass (e.g. phosphate or borate doped). The Applicant noted that U.S. Pat. No. 5,225,925 proposes an amplification fiber that, due to the shape of its gain curve, is particularly adapted for the transmission of a single channel at 1535 nm but is not suitable for WDM transmissions. Moreover, such an amplification fiber is adapted to be pumped, for the excitation of Yb ions, by means of a diode-pumped Nd-doped fiber laser that has the above mentioned disadvantages.
Neither EP 0 803 944 A2 nor U.S. Pat. No. 5,225,925 address amplification by an Er/Yb co-doped optical amplifier of a signal in a wavelength band different from the transmission band around 1550 nm.
An improvement of Er/Yb amplification fibers has been obtained by means of the cladding pumping technique, which consist in pumping the active fiber in an inner cladding region surrounding the core, instead that directly in the core. Cladding pumping is a technique that allows high power broadstripe diodes and diode bars to be employed as efficient, low cost and small dimension pump sources for double-cladding rare earth doped single-mode fibers. Output powers ranging from several hundred milliwatts to several tens of watts may be attained by this technique. A double-cladding Er/Yb fiber pumped by diode arrays at 980 nm is described, for example, in Minelly et al., xe2x80x9cDiode-array pumping of Er3+/Yb3+ co-doped fibre lasers and amplifiersxe2x80x9d, IEEE Photonics Technology Letters, 1993, 5, (3), pp. 301-303. The erbium-ytterbium co-doped scheme enables much higher ground state absorption for erbium in the band about 980 nm than singly-doped erbium fibers, resulting in much shorter optimum length.
The technique of inserting the pump radiation into a portion of the fiber external to the core (which can be identically identified as an inner cladding or an outer core) is also described, for example, in PCT patent application WO 95/10868. This document discloses a fiber optic amplifier comprising a fiber with two concentric cores. Pump power provided by multi-mode sources couples transversely to the outer core (equivalent to an inner cladding) of the fiber through multi-mode fibers and multi-mode optical couplers. The pump power propagates through the outer core and interacts with the inner core to pump active material contained in the inner core. This pumping technique is also described in U.S. Pat. No. 5,291,501, which illustrates a mono-mode optical fiber with doped core and doped inner cladding.
Several methods have also been proposed to increase the number of channels to be transmitted. One way to increase channel numbers is to narrow the channel spacing. However, narrowing channels spacing worsens nonlinear effects such as cross-phase modulation or four wave mixing, and makes accurate wavelength control of the optical transmitters necessary. Applicant has observed that a channel spacing lower than 50 GHz is difficult to achieve in practice due to the above reasons.
Another way to increase the channel number is to widen the usable wavelength bandwidth in the low loss region of the fiber. One key technology is optical amplification in the wavelength region over the conventional 1550 nm transmission band. In particular, the high wavelength band around 1590 nm, in particular between 1565 nm and 1620 nm, is a very promising band for long-distance optical transmissions, in that a very high number of channels can be allocated in that band. If the optical amplifier for the 1565-1620 nm band must deal with a high number of channels, the spectral gain characteristics of such amplifier are fundamental to optimize the system""s performances and costs. The use of the transmission wavelength region around 1590 nm in parallel to the 1530 and 1550 wavelength regions of erbium-doped fiber amplifiers, is attractive and has been recently considered. As an additional advantage, by employing the 1590 nm wavelength region it is possible to use dispersion-shifted fiber (DSF) for WDM transmissions without any degradation caused by four-wave mixing.
Several articles in the patent and non-patent literature address amplification in the high wavelength transmission band (from 1565 nm up to 1620 nm). All these documents consider only erbium-doped fiber amplifiers.
The following documents propose several methods to enlarge the usable bandwidth to the high wavelength transmission band.
U.S. Pat. No. 5,500,764 relates to a SiO2xe2x80x94Al2O3xe2x80x94GeO2 single-mode optical fiber (having a length between 150 m and 200 m) doped with erbium, pumped by 1.55 xcexcm and 1.47 xcexcm optical sources and adapted to amplify optical signals between 1.57 xcexcm and 1.61 xcexcm.
Ono et al., xe2x80x9cGain-Flattened Er3+-Doped Fiber Amplifier for a WDM Signal in the 1.57-1.60 xcexcm Wavelength Regionxe2x80x9d, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 9, No. 5, May 1997, pp. 596-599, disclose a gain-flattened Er3+-doped silica-based fiber amplifier for the 1.58 xcexcm band WDM signal; different fiber lengths were tested and the authors found that 200 m was the optimum length of EDF (Erbium-Doped Fiber) for constructing an EDFA with high gain and low noise.
Masuda et al., xe2x80x9cWideband, gain-flattened, erbium-doped fibre amplifiers with 3 dB bandwidths of  greater than 50 nmxe2x80x9d, ELECTRONICS LETTERS, 5th June 1997, Vol. 33, No. 12, pp. 1070-1072, propose a scheme with two-stage erbium-doped fibres and an intermediate equalizer, obtaining a 52 nm band (1556-1608 nm) for a silicate erbium-doped fiber amplifier and a 50 nm band (1554-1604 nm) for a fluoride erbium-doped fiber amplifier; in the case of a silicate erbium-doped fiber amplifier, the two stages include a 50 m EDF and a 26 m EDF, respectively.
Jolley et al., xe2x80x9cDemonstration of low PMD and negligible multipath interference in an ultra flat broad band EDFA using a highly doped erbium fiberxe2x80x9d, xe2x80x9cOptical Amplifiers and their Applicationsxe2x80x9d Conference, Vail, Colorado, Jul. 27-29 1998, TuD2-1/124-127 proposes a broad band EDFA which amplifies signals in the 1585 nm band using 45 m of erbium fiber and reaching a maximum external power of more than +18.3 dBm at 1570.
The Applicant has observed that a conventional line EDFA adapted to amplify optical signals in the high wavelength band can tipically amplify an optical signal having a total input power of approximately xe2x88x9210 dBm to a maximum power value lower than 19 dBm, i.e. with a maximum gain of approximately 29 dB. A total input power of approximately xe2x88x9210 dBm is a proper reference value, being typical for optical amplifiers in long-distance transmission systems. Lower input power are not recommended in that, although EDFAs have higher gains for low power input signals than for high power input signals, the ASE (amplified spontaneous emission) in this case increases to values such that the signal to noise ratio becomes too low. On the contrary, signal input powers over xe2x88x9210 dBm, obtainable for example to the detriment of transmission fiber length, tends to saturate the gain, leading to an undesirable waste of energy. An optical transmission system using EDFAs and transmitting sixty-four channels between 1575 nm and 1602 nm would provide a maximum power per channel, at the output of the line EDFAs, of about 0.2 dBm and would limit in practice the maximum span length to less than 100 km.
The Applicant has further observed that in an erbium-doped active fiber of a predetermined length, the curve of the gain vs. erbium concentration has an increase up to a maximum, corresponding to an optimum value of erbium concentration, and then a decrease. Higher gains are obtainable only increasing the length of the active region doped with erbium, i.e. increasing the active fiber length. Long-haul WDM optical transmission systems for the high wavelength band using conventional erbium-doped active fibers require fiber lengths of a few hundred meters to reach a relative high gain. Presently, special erbium-doped active fibers having a larger core diameter are considered for use, which allow obtaining a relative high gain with fiber lengths down to 30-40 m.
The Applicant has recently found that, in the 1565-1620 nm band, transmission systems including erbium-ytterbium co-doped amplifiers provide very high performances, in particular they provide higher performances with respect to erbium-only doped optical amplifiers. In the european patent application No. EP98117898 filed on Sep. 22, 1998 in the name of the Applicant, it is proposed an optical amplifying unit including an erbium-ytterbium co-doped fiber amplifier in a single-stage configuration (with bi-directional pumping), or two erbium-ytterbium co-doped fiber amplifiers in a double-stage configuration (with co-propagating pumping or bi-directional pumping), providing high amplification in the 1575-1602 nm wavelength region. To reach very high power gains, the proposed amplifying unit preferably includes an erbium-doped fiber pre-amplifier and at least a double-cladding erbium-ytterbium co-doped fiber amplifier. Double-cladding active fibers allow high pump performances taking advantage of a multi-mode pumping mechanism. The used pump lasers are multi-mode broad-area lasers with an emission wavelength included in the wavelength range 920-980 nm, for example at 920 nm, each adapted to provide a pump power of approximately 400 mW to the active fibers.
In the design of the above described amplifying unit, the Applicant has found that the implementation of a WDM coupler adapted to couple the multimode pump radiation into the double-cladding fiber is critical. Coupling of the multimode pump radiation into the double-cladding fiber is performed preferably by means of micro optic (mirror-type) WDM couplers, which have coupling efficiencies much higher than those of fused fiber WDM couplers. The WDM coupler must be able to couple the pump radiation (in the range 920-980 nm) in the internal cladding of the fiber and the transmitted signal (in the range 1575-1602 nm) in the core. Thus, the coupler must
According to the present invention, the Applicant has found an alternative amplifying unit arrangement adapted to be used in the 1565-1620 nm band and providing advantages over the known amplifying devices. The proposed amplifying unit is particularly suitable for use in a WDM transmission system, preferably as a booster amplifier.
The Applicant has found that, by pumping single-mode and single-cladding active fibre co-doped with Er and Yb by means of a first pump source adapted to excite Er by a first pump radiation and a second pump source adapted to excite Yb by a second pump radiation, a high performance and compact amplifying unit can be achieved.
Preferably, the first pump radiation includes a wavelength between 1465 nm and 1495 nm and is fed to the active fiber in a co-propagating direction (with respect to the transmitted signals) and the second pump radiation includes a wavelength between 1000 nm and 1100 nm and is fed to the active fiber-in a counter-propagating direction (with respect to the transmitted signals).
Preferably, the first pump source is coupled to the active fiber by means of a micro-optic WDM coupler and the second pump source is coupled to the active fiber by means of a fused-fiber WDM coupler.
The amplifying unit of the present invention extends the range of input signals to lower powers, with respect of typical booster units. This feature allows, for example, the design of a transmission system including a device with not negligible losses, for example OADMs (optical Add/drop Multiplexers, i.e. devices for the insertion and the extraction of optical signals to/from the system) or a dispersion compensator, just upstream with respect to the amplifying units. These additional losses can in fact be tolerated without sensible worsening of amplification.
An additional advantage is provided by the use of single-mode couplers to couple the pump radiation to the active fiber, which allows reduced signal losses.
Moreover, the amplifying unit of the present invention has a relatively wide wavelength amplification band extending above 1565 nm, and it is then particularly adapted for use in WDM transmission systems.
According to a first aspect, the present invention relates to an optical transmission system including:
an optical transmitting unit to transmit optical signals,
an optical receiving unit to receive said optical signals,
an optical fiber link optically coupling said transmitting unit to said receiving unit and adapted to convey said optical signals, and
an optical amplifying unit coupled along said link, adapted to amplify said optical signals; said optical amplifying unit comprising:
an input for the input of said optical signals,
an output for the output of said optical signals,
an active fiber codoped with Er and Yb, having a first end optically coupled to said input and a second end optically coupled to said output, for the amplification of said optical signals,
a first and a second pump source for generating a first and, respectively, a second pump radiation, and
a first and a second optical coupler for optically coupling said first pump source and, respectively, said second pump source to said active fibre,
wherein said first pump radiation includes an excitation wavelength for Er and said second pump radiation includes an excitation wavelength for Yb.
Said optical amplifying unit has preferably a wavelength amplification band above 1565 nm.
Advantageously, said first optical coupler is optically coupled to the first end of said active fiber for feeding the first pump radiation to the active fiber in a co-propagating direction with respect to optical signals and said second optical coupler is optically coupled to the second end of said active fiber for feeding the second pump radiation to the active fiber in a counter-propagating direction with respect to optical signals.
The active fiber is preferably a single-cladding fiber and is preferably a single-mode fiber.
The first pump radiation has preferably a wavelength between 1465 nm and 1495 nm and the second pump radiation has preferably a wavelength between 1000 nm and 1100 nm.
The first optical coupler is preferably a micro-optic WDM coupler and the second optical coupler is preferably a fused-fiber WDM coupler.
According to a second aspect, the present invention relates to a method for amplifying optical signals, including the steps of:
feeding the optical signals to an active fiber codoped with Er and Yb; and
optically pumping, during the step of feeding the optical signals, the active fiber;
wherein said step of optically pumping includes feeding to said active fiber a first pump radiation for exciting Er and a second pump radiation for exciting Yb.
Said step of feeding said first pump radiation preferably includes feeding said first pump radiation to the active fiber in a co-propagating direction with respect to optical signals and said step of feeding said second pump radiation preferably includes feeding said second pump radiation to the active fiber in a counter-propagating direction with respect to optical signals.
Said step of feeding to said active fiber a first pump radiation preferably includes feeding to said active fibre an exciting radiation for Er having a wavelength between 1465 nm and 1495 nm.
Said step of feeding to said active fiber a second pump radiation preferably includes feeding to said active fibre an exciting radiation for Yb having a wavelength between 1000 nm and 1100 nm.
Preferably, said active fiber includes a core and a cladding and in said step of feeding to said active fiber a first pump radiation and a second pump radiation includes feeding said first pump radiation and said second pump radiation into the core of said active fiber.
Preferably, said step of feeding the optical signals to the active fiber includes feeding to the active fiber optical signals having wavelengths above 1565 nm.
According to a third aspect, the present invention relates to an optical amplifying unit including:
an input for the input of optical signals,
an output for the output of said optical signals,
an active fiber codoped with Er and Yb, optically connected to said input and said output, and adapted to amplify said optical signals,
a first and a second pump source for generating a first and, respectively,
a second pump radiation, and
a first and a second optical coupler for optically coupling said first pump source and, respectively, said second pump source to said active fibre,
wherein said first pump radiation includes an excitation wavelength for Er and said second pump radiation includes an excitation wavelength for Vb.
Preferably, the excitation wavelength for Er is between 1465 nm and 1495 nm and the excitation wavelength for Yb is between 1000 nm and 1100 nm.
Said first optical coupler is preferably connected between said input and said active fiber for feeding the first pump radiation to the active fiber in a co-propagating direction with respect to optical signals and said second optical coupler is preferably connected between said active fiber and said output for feeding the second pump radiation to the active fiber in a counter-propagating direction with respect to the optical signals.
Said active fiber is preferably a single-cladding and single-mode fiber.
Said first optical coupler is preferably a micro-optic WDM coupler and the second optical coupler is preferably a fused-fiber WDM coupler.
Preferably, said second pump source comprises a fiber laser including a further active fiber and adapted to generate said second pump radiation, and a pump laser source adapted to pump said further active fiber.
Said further active fiber preferably includes a double-cladding fiber. Moreover, said further active fiber preferably includes an optical fiber doped with Vb.
Said fiber laser preferably includes a first and a second Bragg grating written on opposite end portions of said further active fiber, said pump laser source is a broad-area laser diode.
The foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages an suggest additional advantages and purposes of this invention.